1. Field of Invention
This invention relates to electric power systems with protective relays and in particular to such systems in which one, and more typically, in which most, of the protective relays are electromechanical analog devices. The invention provides apparatus for monitoring and controlling the operation of these analog protective relays either alone or simultaneously with digital relays.
2. Background Information
Electric power systems include protective relays which monitor conditions in the system and operate circuit breakers in response to adverse conditions to protect various segments and components in the system. Many of these protective relays are analog devices that have been in place for years. They comprise electromechanical mechanisms which respond to predetermined values of current, and in some relays voltage, and the relationship therebetween to operate electrical contacts. The contacts perform various functions such as tripping a circuit breaker, generating an alarm or providing a signal to another protective relay. For the most part, these analog relays only provide as an output the designed contact operation. They do not provide an indication of the conditions which produced activity in the output contacts. For instance, the principal of operation of a distance measuring relay is that the distance of a fault can be determined by a comparison of the complex impedance of the line derived from the current and voltage at the relay to a reach characteristic. If this comparison indicates that there is a fault within the protection zone of the relay, a trip contact is actuated. However, the analog distance relay does not provide as an output any indication of where the fault is located or what the impedance was, or even what the current and voltage were which caused the trip. Similarly, an overcurrent relay trips in response to detection of a line current which exceeds a set point value, but it does not provide a quantitative indication of the magnitude of current which caused the trip.
Typically, substations in an electric power system have many protective relays. For the most part, the analog relays operate autonomously and without supervision. As mentioned, some relays provide signals to other relays, however, this is done over dedicated lines. Generally, information on the state of the relay contacts and the conditions at the relay is not available to supervisory personnel without dispatching someone to individually inspect the relays. In some instances, remote terminal units (RTU's) provide information on contact status for auxilliary equipment, but not on the relays or breaker trip circuits. In addition, RTU'S require separate lines for each channel of information transmitted to the RTU.
Recently, digital protective relays have been developed for the electric power field. These relays have microcomputers which can be programmed to perform various protection functions. For instance, digital distance relays and overcurrent relays are in use. These relays are capable of providing much more information than the analog relays. For instance, the location of the fault, and the current and voltage at the fault are available in the microcomputer controlled distance relay. Similarly, the current information is available from the digital overcurrent relay. This information has been transmitted to a remote computer, however, to date this has been accomplished by separate RS-232 channels for each relay.
There is a need therefore for a monitoring and control system for an electric power system which provides the capability of remote monitoring and control of protective relays.
There is a particular need for such a system for remotely monitoring and controlling analog protective relays.
There is also a need for such a system for remotely monitoring and controlling power systems incorporating both analog and digital protective relays.
There is, in addition, a need for such a system which provides data on the conditions which caused a protective relay to operate.
There is further need for such a system time tags events detected by the protective relays.